492 IMMUNITY 



toxophorous part of the toxin molecule produces that disturb- 

 ance of the protoplasm which is shown by symptoms of 

 poisoning. If, however, they are in smaller dose, as in the 

 early stages of immunisation, fixation to the protoplasm occurs 

 in the same way ; and as the combination of receptors with 

 toxin is supposed to be of firm nature, the receptors are lost 

 for the purposes of the cell, and the combination E-.-T. (receptor 

 + toxin) is shed off into the blood. The receptors thus lost 

 become replaced by new ones, and when additional toxin 

 molecules are introduced, these new receptors are used up in 

 the same manner as before. As a result of this repeated loss 

 the regeneration of the receptors becomes an over-regeneration, 

 and the receptors formed in excess appear in the free condition 

 in the blood stream and then constitute antitoxin molecules. 

 There are thus three factors in the process, namely, (1) fixation 

 of toxin, (2) over-production of receptors, (3) setting free of 

 receptors produced in excess. Accordingly these receptors 

 which, when forming part of the cell protoplasm, anchor the 

 toxin to the cell, and thus are essential to the occurrence of 

 toxic phenomena, in the free condition unite with the toxin, and 

 thus prevent the toxin from combining with the cells and exert- 

 ing a pathogenic action. The three orders of receptors, when 

 separated from the cells, thus give the three kinds of anti- 

 substances. Ehrlich does not state what cells are specially 

 concerned in the production of anti-substances, but from what 

 has been stated it is manifest that any cell which fixes a toxin 

 molecule, for example, is potentially a source of antitoxin. 

 Cells, to whose disturbance, resulting from the fixation of toxin, 

 characteristic symptoms of poisoning are due, will thus be 

 sources of antitoxin, e.g. cells of the nervous system in the case 

 of tetanus, though the cells not so seriously affected by toxin 

 fixation may act in the same way. The experimental investiga- 

 tion of the source of antitoxins has, however, yielded little result, 

 and no definite statement can be made on the subject. 



When we come to consider how far Ehrlich's theory is in 

 harmony with known facts, we find that there is much in its 

 favour. In the first place, it explains the difference between 

 active and passive immunity, e.g. difference in duration, etc. ; in 

 the former the cells have acquired the habit of discharging anti- 

 substances, in the latter the anti-substances are simply present 

 as the result of direct transference. It is also in harmony with 

 the action of antitoxins, etc., as detailed above, and especially it 

 affords an explanation of the multiplicity of anti-substances. 

 For, if we take the case of antitoxins, we see that this depends 



